1. Field of the Invention
The present invention relates to an image processing method for processing an image in the form of digital signal and an apparatus therefor, and more particularly to an image processing method for reproducing an image with high quality and an apparatus therefor.
2. Related Background Art
There is already known the error dispersion method as a binary digitizing method for reproducing intermediate tone in digital printers or digital facsimile apparatus. In said error dispersion method, the difference in density between the original input image and the output image after processing is calculated for each pixel, and the error obtained in said calculation is dispersed into surrounding pixels with predetermined weighting. Said error dispersion method was presented by R.W. Floyd and L. Steinberg in "An Adaptive Algorithm for Spatial Gray Scale", SID 75 Digest (1976).
Such error dispersion method can conserve the density of the input image in the output image, and, lacking periodicity in the processing, is free from Moire fringe patterns encountered in other binary digitizing methods such as the dither method or density pattern method, but is associated with the drawbacks of particular fringe patterns in the uniform density area (nonedge area) of the image or granular noises due to dispersed dots particularly in the highlight area of the image, thus leading to the deterioration of image quality.
For avoiding these drawbacks, U.S. Pat. No. 4,876,610 proposes a method of discriminating the characteristic of image and dispersing the error according to the result of said discrimination. Also, U.S. Pat. No. 4,878,125 proposes a method of varying the ratio of dispersion of error into the surrounding pixels according to the amount and direction of edge present in the image, and U.S. patent application Ser. No. 145,593 filed Jan. 19, 1988, proposes a method of dividing the original image into blocks, digitizing the image in each block and dispersing the error in said digitization into surrounding blocks. Both mentioned patents and the patent application are assigned in common with the present invention.